“All unrighteousness is sin,” 1 John 5:17. Every violation of the divine law is sin, so we can answer this question by looking at God’s laws on marriage. 

The divine law blessed marriage between one man and one woman from the beginning and intended it to be permanent. God gave Adam one helper, not helpers plural.  “…a man shall…be joined to his wife; and they shall become one flesh.” This divine arrangement has never changed (Genesis 2:20-24,27-28 NASB).

When God started civilization over, He chose four couples, “Noah, and Shem, and Ham, and Japheth, the sons of Noah, and Noah’s wife, and the three wives of his sons with them…” (Genesis 6:8-10; 7:7,9,13,15).

Nevertheless, in the Old Testament polygamy was permitted for a time (Genesis 30:26). For example Abraham, Jacob, David and Solomon all had multiple wives. Note these polygamous unions resulted in disfunction and unhappiness 100 percent of the time! God may have tolerated man to engage in these unsanctioned behaviors in order to learn that God’s arrangements are always the best. 

Recall that Hagar was a concubine given to Abraham by his wife, Sarah. God provided for Hagar. An angel told her, “…You will give birth to a son, Ishmael…for the LORD has heard of your misery.” But it caused great unhappiness for Abraham, Sarah, Hagar and the children, and God sent Hagar away. God comforted Abraham in his sadness. Then God said to Abraham, “And as for Ishmael, I have heard you: I will surely bless him…I will make him into a great nation” (Genesis 16:11, 17:18-20 NIV).     

God told Israel, the king “must not take many wives, or his heart will be led astray,” Deuteronomy 17:14,17 NIV. Verses 18-20 go on to say this was “the Law.” A study of the lives of men like King David and King Solomon will show that many of their problems were due to their polygamous relationships. Sorrow always follows sin and is the consequence for not obeying God’s Law.

Jesus often began his teachings by saying, “You have heard it was said to them of old time…but I say to you…”  In this way, Jesus supported the Old Testament and God’s original divine arrangement. Jesus’ teachings on marriage are in Matthew 5:27-32, Matthew 19:3-12, Mark 10:1-12 and Luke 16:15-18. His teachings do not include polygamy as a choice. But the habit of polygamy was still practiced at that time. Consequently, Paul spoke plainly to the churches, telling them that their pastors must be the husband of only one wife (1 Timothy 3:2,12). There are NO Christian examples of polygamy.  Marriages were only one man + one woman.  We follow the Christian example.  

The marriage relationship illustrates the relationship between Jesus and his church, his most faithful followers. “Therefore, a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh. This mystery is profound, and I am saying that it refers to Christ and the church” (Ephesians 5:31,32). Whenever the Bible identifies something as a copy or shadow of heavenly things, it is important to keep the picture carefully! Christian marriages are a picture of the holy, loving and beautiful relationship Jesus has with his church. Jesus has one bride. The bride of Christ has one husband.

Additional Resource:
Christian Questions Podcast
Episode #1241: “Is It a Sin if I…? (Part II)”
Discussing sex and sin-related questions using biblical principles
Preview Video
CQ Rewind Show Notes